Drilling on Midway Atoll, Hawaii

Ladd, H; Tracey, J; Gross, M

Two holes drilled through reef sediments into basalt have established a geologic section through the Miocene. Midway was built above the sea by flows that were weathered and partially truncated in pre-Miocene time. After submergence, volcanic clays were reworked and covered by limestones. Overall submergence was interrupted at least twice by emergence. The limestones have been leached, recrystallized, and partially dolomitized. [Geologic history] At Midway, basaltic lavas were poured out to build a mound rising nearly 3 miles above the ocean floor. The exact date at which the mound reached the surface is not known, but the subaerial or shallow-water lava flows that cap it were weathered and at least partially truncated by wave action prior to the Miocene. In the early Miocene (Tertiary e) the northern part of the eroded mound was covered by swamps. The mound subsided, and the swamps were covered by shallow marine waters in which ostracods and smaller foraminifera lived on a muddy bottom. Altogether, 170 feet of lignitic beds and clays were accumulated. As the waters cleared, reef corals and calcareous algae became established on the reworked volcanic clays, moving in, probably, from a more northerly site to windward. As subsidence continued, 500 feet of reef limestone were accumulated, apparently under lagoonal conditions. Water circulation may at times have been restricted, permitting the formation of dolomitizing solutions. At or near the end of Tertiary e, the reef emerged, and the entire limestone section stood above water during Tertiary f. The limestones were thoroughly leached and recrystallized. Resubmergence followed in the late Miocene (Tertiary g), and more than 100 feet of shallow-water lagoonal limestones were deposited. These deposits covered the altered limestones on the north side of the atoll and the higher area to the south. This interval of limestone deposition appears to have continued into the Pleistocene. Altogether, about 250 feet of post-Miocene beds were accumulated before there was another major interruption. This change·like the one that interrupted the early Miocene sequence· was caused by emergence of the atoll. In this instance the withdrawal may have been due to the eustatic lowering of the sea caused by Pleistocene glaciation. It may be possible to tie the 200-foot solution unconformity of Midway to the extensive submarine platform lying 180 feet below sea level off the island of Molokai and elsewhere in Hawaii. During this period of emergence the newly formed limestones were leached and recrystallized. When subsidence recommenced, the altered limestones were covered by about 200 feet of reef and lagoonal sediments. These beds have not been emergent for long periods and exhibit no evidence of leaching or extensive recrystallization. The last recorded event was the growth of a now-emergent reef, remnants of which are found on most of Midway's rim (Fig. 3). This reef apparently flourished when the sea stood several feet higher than it does now. The earliest date obtained in several carbon-14 determinations made on the exposed reef rock by Meyer Rubin of the Geological Survey was 2400 years ago. The old reef is now eroded and no part now rises more than 3 feet above high-tide level, but it probably represents the Recent negative shift in sea level of 5 to 6 feet, evidence for which, from many parts of the Pacific, has been described.

Drilling on Midway Atoll, Hawaii

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